This invention relates generally to gas producing charges and more particularly to double base propellants exhibiting very low dependence of burning rates on conditioned temperature and also exhibiting plateau and/or mesa burning rate behavior at high burning rates and pressures.
Relatively recently, a great deal of interest has emerged in the development of plateau and mesa type propellants. These propellants are characterized by an unusual pressure-burning rate relationship, this relationship defining the plateau and mesa characteristics. The pressure-burning rate relationship is defined by the slope n, or pressure exponent, of the curve produced by the logarithmic graph of the burning rate of propellant plotted against pressure. This relationship, i.e. between the pressure at which a propellant burns and its burning rate is mathematically expressed as r = cp.sup.n where r is the burning rate, p is the pressure and c and n are constants characteristic of the particular propellant. The equation can be reexpressed as log r = n log p + log c. Thus, a plot of log r against log p for a conventional propellant gives a straight line which has a slope n, i.e., representative of a progressive increase in burning rate for each increase in pressure. However, in the case of plateau type propellants, the pressure exponent n becomes zero in a certain region of pressure. Such propellants at a given temperature give a steady burning rate within the region and, consequently, a steady thrust. In the case of mesa type propellants the pressure exponent n is negative in certain regions of pressure so that the burning rate decreases slightly with increasing pressure. There are a number of well known advantages to propellants exhibiting this "plateau" and "mesa" phenomena. For example, with conventional double base propellants, pressures build up rapidly and a thick-walled chamber is needed to contain the propellant tending to make rockets thus powered heavy and poor in ballistic performance. This disadvantage is overcome by the plateau and mesa type propellants. Further, in the case of mesa type propellants, there is an inherent tendency for overlapping of rate-pressure relationships at various temperatures as illustrated by logarithmic graphs of the relationships, that is, in certain regions of pressure the burning rate of a propellant for firings at low temperature may be actually higher than the burning rate for firings at high temperature. Additionally, the variation in performance with change in temperature for mesa type propellants is negligible and in some cases there is none at all. Examples of such mesa type propellants which possess this further advantage, that is, burning rates which are substantially temperature insensitive over wide ranges of operating pressures are described in U.S. Pat. No. 3,138,499 by Camp et al. In light of the above described desirable characteristics of plateau and mesa type propellants, it has now become desirable to further augment these advantages by formulating propellants which exhibit plateau and/or mesa regions at very high useable pressures and, concomitantly, exhibit very fast burning rates. Such propellants will provide for higher specific impulses and, in view of the fast burning rates, will allow greater efficiency and easier handling of, for example, rocket launchers and other combat weapons.
These plateau and mesa phenomena are due to the substitution of certain amounts of ballistic modifier compounds for the nitroglycerin and nitrocellulose of the basic propellant formulation. Due to the high calorific values attributed to nitrocellulose and nitroglycerin the greater the substitution for these materials with ballistic modifiers (in order to attain higher operating pressures) the lower becomes the heat of explosion of the mixture. It is therefore also desirable to provide more efficient plateau and mesa type propellants, that is those which utilize less ballistic modifier, yet which provide for higher useable operating pressures.
In addition to the above desirable properties which are characteristic of plateau and mesa type propellants, a suitable propellant should possess other requisite properties. For example, it should have good storage stability characteristics and should lend itself readily to a feasible mass production process.